It is generally known to treat the surfaces of metals, such as zinc, cadmium, or aluminum with aqueous hexavalent chromium solutions which contain chemicals that dissolve the surface of the metal and form insoluble films known as “chromate conversion coatings.” These coatings, which contain hexavalent chromium, are corrosion resistant and protect the metal from various elements which cause corrosion. In addition, it is known that hexavalent chromate conversion coatings generally have good paint bonding characteristics and, therefore, provide an excellent base for paint or other finishes.
Although the aforementioned coatings enhance corrosion resistance and paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the hexavalent chromium constituent. This is a serious problem from two viewpoints, one being the handling of the solution by operators and the other, the disposal of the used solution. Therefore, it is highly desirable to have coatings which are free of, or substantially free of, hexavalent chromium, but at the same time capable of imparting corrosion resistance and paint bonding properties which are comparable to those imparted by conventional hexavalent chromium coatings.
Of particular interest is the use of hexavalent chromate conversion coatings on aircraft aluminum alloys due to the excellent corrosion resistance and the ability to serve as an effective base for paint. The baths used to develop these coatings contain hexavalent chromates, and it is the residual hexavalent chromates in the coating that is largely responsible for the high degree of corrosion inhibition. However, these same hexavalent chromates are toxic and their presence in waste water effluents is severely restricted. It would, therefore, be desirable to provide a composition for coating aluminum and its alloys, and for sealing of anodized aluminum, utilizing less hazardous chemicals that could serve as an alternative to the toxic hexavalent chromate coatings. There has been a significant unmet need in the coating industry to provide conversion coatings that contain little or no hexavalent chromium, but which still provide corrosion resistance and paint bonding that is comparable to the prior art hexavalent chromium containing conversion coatings.